This invention, generally, relates to motor aided systems and, more particularly, to motor aided power steering systems.
Drivers steer automobiles through a system of gears and linkages that transmit the turning motion of a steering wheel to a vehicle""s front wheels. As vehicle designs shift weight to the front wheels to improve riding comfort and vehicle handling, more effort is needed to turn the front wheels and provide sufficient torque to overcome the drag that exists between the front wheels and the road.
Power steering systems are designed to reduce steering effort and improve maneuverability of the vehicle. Some vehicles use engine driven hydraulics to amplify the torque applied by the steering wheel to the front wheels. In such a system, a mechanically or an electrically driven pump maintains a hydraulic fluid, such as oil, under pressure. The rotation of the steering wheel actuates a valve, which supplies or drains fluid to a power cylinder as needed to reduce the steering effort necessary to turn the wheels. A problem exists with the hydraulic system, however, in that, if the hydraulic system fails, the driver must steer the fluid that couples the steering wheel to the front wheels, in addition to overcoming the drag between the wheels and the road. Moreover, the hydraulic system necessitates extra parts such as belts, hoses and pumps because the hydraulic system cannot directly couple to the steering shaft.
To avoid problems that accompany the hydraulic system, some vehicles directly couple an electric motor to the steering shaft through a gearbox. In one example, a three-phase alternating current (ac) motor is used and the three-phase voltage required to operate the motor from the vehicle""s direct current (dc) battery is obtained using a three-phase inverter connected between the dc battery and the motor. For safety reasons, relays are used to ensure fail-safe operation in case of the inverter faults. For example, one relay disconnects the battery from the inverter""s switches while another relay closes to form a neutral point of the three-phase motor.